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Build A Laser Light Show How would you like a laser light show for your next party? You can build your own and it doesn’t need to be a large complex beast with a big laser and so on. This laser display is quite tiny yet it can project a very satisfying display onto the walls of your lounge room. By LEO SIMPSON W E DESCRIBED a motorised laser light show in the May 1996 issue but this was a big and bulky beast with a 100mW blue Argon or a 10mW Helium-Neon gas laser, a big power supply and special circuitry for the motor-driven deflection mirrors. Both were very effective and suitable for large venues but they were probably overkill for anyone who just wants a small laser display for parties in their home. By contrast, this laser light show is very compact and fits into a small instrument case on a swivel stand. Inside, it has a tiny semiconductor 82  Silicon Chip laser and its beam is deflected by two mirrors driven by equally tiny motors. Have a look at the photos and you will see that there is almost nothing to it. This display system employs two DC motors with mirrors on their shafts to deflect the laser beam. The motor shafts do not rotate but simply vibrate around a central tethered position. The level of vibration depends on the level of audio signal applied to the input. Two motors are provided to deflect the laser beam in the horizontal and vertical directions. And since a typical audio signal is more or less random, the resulting display is endlessly variable, with the beam deflection being proportional to the signal amplitude. The motors and their mirrors are angled in such a way as to provide optimum deflection of the laser beam. Due to the mass of the motor armatures and the mirrors attached to the shafts, these small DC motors only produce a useful response to signals of no more than a few hundred Hertz. Bass frequencies are quite effective but midrange and high audio frequencies do not produce any useful beam deflection. But the available response still produces a very useful and interesting range of laser patterns. While the range of mirror deflection is set by the ampli­ tude of the low frequency audio signals, the central position of each mirror is fixed by a small strip of polycarbonate film between the motor shaft and body. Circuit description Two audio signals are needed for this laser drive circuit but since we can only use bass to lower midrange Fig.1: the circuit has an electret microphone to pick up music signals and these are used to drive two small DC motors. frequencies there is really only one signal present in typical program mate­ rial, whether it is stereo or mono. As an aside, most stereo tapes and CDs have very little separation between the left and right audio signals in the bass region, hence there is really only one bass signal. This circuit gets around that problem by feeding one of the motors with straight bass while the second motor is fed with a signal derived from the mid­ range to treble part of the spectrum. This signal is rectified and filtered. In effect, the derived signal is the rate of change (or envelope) of the midrange to treble signal. The resultant pattern produced by the laser simply depends on the sound picked up by the an electret microphone. Dif­ferent types of music and sounds tend to generate their own unique patterns and you may find yourself playing music chosen more with an eye to the laser pattern rather than how it sounds. Looking at the lefthand side of the circuit (Fig.1), resistors R1 & R2 and capacitor C2 provide a decoupled supply voltage to the electret microphone. The output from the electret microphone is coupled via a 10µF capacitor to the first amplifier stage involv­ing op amp IC1a. This is configured as an inverting Liven up your next party with this compact laser light show. Use it to produce endless patterns on your living room walls. stage with a gain depending on the setting of VR1. This can range from unity to about 100. Following IC1a, the amplified elec­ tret signal is fed via two paths. Path number one is via a low pass filter consisting of resistors R6 & R7, togeth­ er with capacitors C5 & C6. This filter effectively blocks frequencies above about 350Hz before they are fed to op amp IC2b which has a fixed gain of 10. The output from this stage is applied via VR3 to IC4, an LM380 power amplifier, and this is used to drive one of the deflection motors. Path number two from IC1a is via a April 1998  83 Fig.2: component overlay diagram for the PC board. Take care to ensure that all polarised parts are correctly oriented. high pass filter con­sisting of capacitors C3 & C4 and resistors R5 & R8 and this effectively blocks frequencies below about 350Hz. This is the other half of the audio spectrum from IC1a and this is applied to op amp IC2a which also has a fixed gain of 10. IC2a’s output is fed to a “diode pump” rectifier consisting of diodes D1 & D2 and capacitors C7 & C9. The rectifier output represents the “rate of change of the midrange signal” and this signal is applied via potentiometer VR2 to IC3, another LM380 power amplifier, and this drives the second deflection motor. Power supply Power for the circuit is provided by a 13.8V DC plugpack with a capacity of 300mA or more. Op amp IC1b, zener diode ZD1 and their associated components are used to derive a 12V regulated supply, which is used as a bias voltage for op amp stages IC1a, IC2a & IC2b. A 7805 3-terminal regulator provides a fixed +5V rail for the solid state laser module. Also shown on the circuit is a DPST switch (S2) which makes provision to drive the motor deflection circuits from a stereo amplifier (ext). Construction Fig.3: use this diagram when wiring up your laser display. Power comes from a 13.8V DC plugpack supply. 84  Silicon Chip All the circuitry, apart from the solid state laser module and the 3-terminal regulator, is mounted on a PC board measuring 96 x 47mm. This board is divided into two sections, one involving IC1 & IC2 while the other accommodates the two power amplifiers, IC3 & IC4. Two links between the two sections allow you to add the DPST switch S2. Our prototype does not include this and provided the electret microphone picks up adequate audio signal, it is more convenient without any need for audio signal cables. The first task in assembling this project is to assemble the PC board and this is quite straightforward since it comes with the component overlay screen-printed on top – see Fig.2. Insert all the smaller components first, followed by the trim­pots, electrolytic capacitors and lastly, the ICs. IC sockets can be regarded as optional. When the board assembly is complete, connect up the 13.8V DC power supply and the two motors. With no signal, nothing much happens. However, when you speak or blow into the electret micro­phone, the motor shafts Parts List 1 plastic case, 154 x 65 x 158mm 1 swivel stand to suit 2 miniature DC motors 2 small aluminised glass mirrors 1 solid state laser module 1 finned heatsink to suit 3terminal regulator 1 PC board, 96 x 47mm 1 13.8V DC plugpack with 2.1mm DC plug 1 DC socket to suit 1 electret microphone insert 1 SPST miniature toggle switch (S1) 1 1MΩ horizontal trimpot (VR1) 2 100kΩ horizontal trimpots (VR2) The laser beam is deflected by the two motor-driven mirrors (top) in response to audio signals from your music system. The electret microphone picks up the audio signal and feeds it to the circuit via a shielded cable. should vibrate rapidly back and forth. Case work The next step is to wire up the 3-terminal regulator. This is mounted on a small finned heatsink and the external capacitors are soldered between its three legs. The two power diodes are wired in series with the output terminal and all connections are then secured with small-diameter heatshrink tubing. You will need to drill a large hole of between 20mm and 30mm in the front panel for the laser to be aimed through. Ideally you should use a hole punch for this job but if you don’t have one, you can drill a smaller diameter hole and then ream or neatly file it out to size. On the rear panel, you will need holes to mount the 3-terminal regulator, DC socket and power switch and a small hole for the electret microphone cable. On the base of the case, you will need to drill holes to mount the PC board, the two motors, the solid state laser module and the swivel stand. Fig.3 shows how all the wiring should be run to the motors, PC board and so on and the photographs give a further guide to the orientation of the motors and laser module. Motor mounting Earlier on, we implied that one motor is used for vertical deflection and the other is used for horizontal deflection of the laser beam. They could be arranged to do this but it is Semiconductors 2 TL072 dual Fet-input op amps (IC1, IC2) 2 LM380N power amplifiers (IC3, IC4) 1 7805 5V 3-terminal regulator (REG1) 1 12V 400mW zener diode (ZD1) 2 1N60 small signal diodes (D1,D2) 2 G1G rectifier diodes (D3,D4) Capacitors 4 100µF 16VW or 25VW PC electrolytic 2 47µF 16VW PC electrolytic 5 10µF 16VW or 25VW PC electrolytic 5 0.1µF monolithic or MKT polyester 4 .0033µF monolithic or MKT polyester 2 680pF ceramic Resistors (0.25W, 1% or 5%) 2 1MΩ 1 5.6kΩ 8 100kΩ 1 4.7kΩ 3 10kΩ 2 2.7Ω Miscellaneous Motor brackets, laser module bracket, polycarbonate strip, 5minute epoxy adhesive, shielded cable, solder. far more convenient to mount each motor with its major axis at 45 degrees to the horizontal and angled in such a way that the laser bounces off one mirror to the next and then shines out through the front panel hole. April 1998  85 This close-up view shows how the two mirrors are glued and tethered to the motors. The tethers allow the mirrors to deflect the laser beam by about ±30°, which is enough to produce an interesting pattern The 3-terminal regulator and its associated parts (including the heatsink) are mounted on the rear panel. Where To Buy The Parts All parts for this project are available from Oatley Electronics who own the design copyright. Their address is PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563; fax (02) 9584 3561. The prices are as follows: PC board plus on-board parts, motors, mirrors, electret microphone....... $44.00 5mW 650nm laser module........................................................................ $25.00 13.8V 1A DC plugpack.............................................................................. $12.00 Complete kit, including all above parts, 3-terminal regulator & case........ $85.00 86  Silicon Chip Before you can mount the motors, you need to attach the mirrors to the shafts and fit them with tethers. The two small mirrors supplied have aluminium metallisation on one side and this side must be used to reflect the laser beam. If the glass side of the mirror is used to deflect the beam, the effect will be to defocus it. You can glue the mirrors to the mirror shafts using 5-minute epoxy adhesive. Make sure you don’t get any adhesive on the aluminium side of the mirrors. Once the mirrors are glued in place, you can attach the tethers between the mirrors and the motor cases. The tethers are strips of polycarbonate film and should be long enough to let the mirrors be deflected by a maxi­mum of ±30°. This is more than enough to give good deflection of the laser beam and will not unduly load the motors. Again, the polycarbonate tethers can be glued in place with 5-minute epoxy adhesive. Our prototype had small metal brackets soldered to the motors and these were then screwed to the base of the case. The laser module was mounted by holding its lens assembly with a circular clamp attached to a vertical bracket. In practice, you could mount the laser as shown in the photos but with the baseplate screw not tightened. Then you could position and angle the motors so that the laser can be aimed and deflected as required. Once you are satisfied with the laser beam deflection, the motor positions can be marked, holes drilled in the baseplate and then the motors can be secured. Do not mount the electret microphone inside the case. If this is done, it will inevitably pick up the vibration of the motors and the whole system will then oscillate at a low frequen­cy. This is the reason for connecting the electret microphone via a length of shielded cable. That way, it can pick up sound from your music system rather than from the motors. The electret microphone insert used in our prototype has the shielded cable attached directly to its rear lugs and then it was neatly shrouded with heatshrink tubing to anchor and provide stress relief for the cable. Before you can put the Laser Light Show to use, you will need to adjust trimpot VR1 for adequate gain from the electret and then set VR2 and VR3 for SC optimum mirror deflection.